Without limiting the scope of the disclosure, this background is described in connection with external fixation devices and specifically connection struts and rods. Generally, external fixation devices are commonly used in a variety of surgical procedures including limb lengthening and deformity correction. The process involves a rigid framework comprising several rings that are placed externally around the limb and attached to bone segments using wires and half pins inserted into the bone segments and connected to the related section of the external rigid framework. The opposite rings of the rigid framework are interconnected by either threaded or telescopic rods directly or in conjunction with uni-planar or multi-planar hinges, which allows the surgeon to adjust position of the rings relative to each other longitudinally, rotationally, horizontally or angularly over a period of time.
For example, in limb lengthening, the bone is surgically divided into two segments and wires and half pins are inserted into bone segments above and below the surgical bone cut and attached to rings of a rigid framework interconnected by struts or telescopic connection rods. The rigid framework is used to gradually push the two bone segments apart longitudinally over a period of time (e.g., one millimeter a day). This allows the bone to gradually form in the gap between bone segments created by this distraction technique. Once the desired amount of lengthening is achieved (e.g., 5-6 cm), the external apparatus is stabilized into a fixed position and left on the bone segments until complete mineralization of the newly formed bone (e.g., 3-6 months, depending on the nature of pathology and amount of lengthening).
Similarly, in deformity correction, the bone is surgically divided (usually at the apex of the deformity) into two segments and wires and half pins are inserted into bone segments above and below the surgical bone cut and attached to rings of a rigid framework. Opposite rings of the rigid framework are connected together by threaded rods with attached uni-planar or multi-planar hinges and angular distractor that is used to gradually push the two bone segments apart angularly over a period of time.
One common fixation device is a circular metal structure known as the Ilizarov Apparatus. The Ilizarov apparatus, when used for limb lengthening or deformity correction, consists of several rings or arches that are placed externally around the limb and attached to surgically separated bone segments using wires and half pins. For limb lengthening, the opposite rings are interconnected directly by three or four threaded or telescopic rods that are regularly adjusted in length and allowed for gradual separation of bone segments longitudinally. For angular deformity correction, the opposite rings of the Ilizarov apparatus are connected by a pair of hinges that provide an axis of rotation for bone segments and an angular distractor that gradually pushes two rings and associated bone segments apart.
Another common external fixation device is the Taylor Spatial Frame, which is a hexapod type external fixation device based on a Stewart platform but shares many components and features of the Ilizarov apparatus. The Taylor Spatial Frame consists of two external fixation rings attached to bone segments by wires and half pins and connected together by 6 telescopic struts with multi-planar hinges located at both ends of the strut. Each strut may be lengthened or shortened as necessary to either pull two interconnected ring segments towards each other or push them apart. Adjustment of strut length allows manipulating with bone segments acutely or gradually in 6 axes (e.g., lengthening/shortening external/internal rotation, anterior/posterior horizontal translation, medial/lateral horizontal translation, anterior/posterior angular translation, and medial/lateral angular translation) to perform limb lengthening and correct angular, translational and rotational deformities simultaneously.
The Taylor Spatial Frame includes a plurality of struts interconnecting a pair of rings. The wires, half pins, struts and other connection and assembly elements of the frame are connected to the rings via apertures defined therein. All of those apertures or holes are located on the same ring surfaces extending through the upper ring surface to the lower ring surface. This creates a positioning problem for wire and half pin attachment and placement of the additional connection rods and assembly elements due to the competition for holes in the fixation ring of the frame or the wires and pins interfering with the connections of the struts to the holes.
Each strut of the Taylor Spatial Frame has a threaded rod partially disposed inside of a hollow shaft, and the hollow shaft includes an adjustment nut that mates with the threaded rod. To effect either a coarse adjustment (rapid strut length adjustment) or a fine adjustment (gradual strut length adjustment) to the length of the strut, the same threaded rod is pulled out or pushed in relative to the hollow shaft. Because the threaded rod has a finite length, however, using the same threaded rod for rapid strut length adjustment limits the total amount of threaded rod available for gradual strut length adjustment during, for example, limb lengthening and deformity correction. As a result, a time consuming exchange of the struts for longer ones during the treatment is required.
Additionally, the replacement or removal of a strut from the Taylor Spatial Frame during the course of treatment is impossible without using external support or other stabilization mechanism to support the rest of frame. The struts of the Taylor Spatial Frame are required to be connected at the top or bottom of the rings. Such connections require the use of either ball joints in the holes of the rings or universal joints extending from the top or bottom surfaces of the rings. The Taylor Spatial Frame, however, does not include any locking mechanism for temporarily locking the universal joints or ball joints in their orientation. As a result, if one strut is removed from the frame, it would become unstable and collapse.
The universal joints and ball joints of the Taylor Spatial Frame also make strut length adjustments inaccurate and inconvenient. The joints of the Taylor Spatial Frame allow each strut to have three degrees of rotational freedom, including an undesirable degree of freedom for axial rotation. As such, when the adjustment nut of a strut is rotated axially to effect a strut length adjustment as discussed above, the entire strut would also rotate axially, thereby keeping the threaded rod of the strut from translating relative to the hollow shaft of the strut. To allow the threaded rod to translate as desired, one would have to use a second hand or a vice to secure the hollow shaft from axial rotation. Absent such an inconvenient step, strut length adjustments would be inaccurate.
Both types of joints have additional drawbacks. One drawback for using ball joints is that there is insufficient spatial clearance for the ball joints to effect some adjustments to the orientation of the struts and the rings. A drawback for using universal joints is the decrease in the functional length of the struts due to the space taken up by the length of the universal joints. Another drawback is the instability caused by universal joints. The universal joints each include at least one hinge connection, in which a pivot pin is commonly and loosely held by two rotating parts that are designed to rotate about the longitudinal axis defined by the pivoting pin. Due to the loose coupling between the rotating parts and the pivoting pin, a degree of undesirable translational movement is inherent in the design of the hinge connection and introduces instability to the Taylor Spatial Frame.